Wireless communication systems include long term evolution (LTE), LTE-A, and LTE-A beyond systems. Typically, in a modern wireless communications system, there is a plurality of NodeBs (NBs) (also commonly referred to as base stations, communications controllers, or eNodeBs (enhanced NBs), and so on, and may even include network points using different radio access technologies (RATs) such as high speed packet access (HSPA) NBs and WiFi access points). A NodeB may be associated with a point or multiple points, and a cell may include a point or multiple points, with each point having a single or multiple antennas. A point may also correspond to multiple cells operating in multiple component carriers. The eNodeBs are interconnected with each other by means of an X2 interface. The eNodeBs are also connected by means of an S1 interface to a Mobility Management Entity (MME) and to a Serving Gateway (S-GW). Additionally, a cell or NB may be serving a number of users (also commonly referred to as User Equipment (UE), mobile stations, terminals, and so forth) over a period of time.
In a downlink transmission, reference signals (RSs) and other signals such as a data channel (physical downlink shared channel (PDSCH)), a control channel (physical downlink control channel (PDCCH)), and an enhanced PDCCH (ePDCCH) are orthogonal and multiplexed in different resource elements in the time-frequency domain. In an uplink transmission, physical uplink shared channel (PUSCH) and physical uplink control channel (PUCCH) are orthogonal and multiplexed in different time-frequency resources.
Generally, to enable any data channels in either uplink (UL) or downlink (DL) transmissions such as PDSCH or PUSCH of an LTE-A system, reference signals are transmitted. There are reference signals for a UE to perform channel/signal estimation/measurements for demodulation of PDCCH and other common channels as well as for some measurements and feedback (known as the Common/Cell-specific Reference Signal (CRS) inherited from the Rel-8/9 specification of E-UTRA). A Dedicated/De-modulation reference signal (DMRS) can be transmitted together with the PDSCH channel in Rel-10 of E-UTRA. DMRS is used for channel estimation during PDSCH demodulation. In Rel-10, Channel State Information Reference Signal (CSI-RS) is introduced in addition to CRS and DMRS. CSI-RS is used for Rel-10 UEs to measure the channel status, especially for multiple antennas cases. PMI/CQI/RI and other feedback information may be based on the measurement of CSI-RS for Rel-10 and beyond UE. PMI is the precoding matrix indicator, CQI is the channel quality indicator, and RI is the rank indicator of the precoding matrix. CSI-RS in Rel-10 can support up to eight transmission antennas while CRS can only support maximal 4 transmission antennas in Rel-8/9. The number of CSI-RS antenna ports can be 1, 2, 4, and 8. In addition, to support the same number of antenna ports, CSI-RS has much lower overhead due to its low density in time and frequency.
A heterogeneous network (HetNet) comprises high power macro points and various lower power points that generally may share the same communication resources. The lower power points may include, but are not limited to, picos, remote radio heads (RRHs), femtos (or home eNodeBs (HeNodeBs)), access points (APs), distributed antennas (DAS), relays, and near field communication points.
A network also may include several component carriers operating in different frequency bands. High frequency bands generally have a high pathloss over distance so they are more suitable to serve a relatively smaller area, such as used for high throughput purpose for nearby UEs. Low frequency bands generally have low pathloss over distance so they are more suitable to serve a relatively large area, such as used for providing coverage.